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Light Wave Systems
Dr Manoj Kumar
Professor & Head
Department of ECE
DAVIET,Jalandhar
Overview
• In this section we cover point-to-point digital
transmission link design issues (Ch8):
– Link power budget calculations
– Link rise time calculations
A link should satisfy both these budgets
Fig. 8-1: Simple point-to-point link
This p-p link forms the basis for examining
more complex systems
System Requirements
1. Transmission Distance
2. Data Rate for a given BER
Selecting the Fiber
Bit rate and distance are the major factors
Other factors to consider: attenuation (depends on?)
and distance-bandwidth product (depends on?) cost
of the connectors, splicing etc.
Then decide
• Multimode or single mode
• Step or graded index fiber
Selecting the
Optical Source
• Emission wavelength
• Spectral line width
(FWHM) and number
of modes
• Output power
• Stability
• Emission pattern
• Effective radiating area
LED
LASER
Selecting the detector
• Type of detector
– APD: High sensitivity but complex, high bias voltage
(40V or more) and expensive
– PIN: Simpler, thermally stable, low bias voltage (5V or
less) and less expensive
• Responsivity (that depends on the avalanche gain
& quantum efficiency)
• Operating wavelength and spectral selectivity
• Speed (capacitance) and photosensitive area
• Sensitivity (depends on noise and gain)
Typical bit rates at different
wavelengths
Wavelength
LED Systems
LASER Systems.
800-900 nm
150 Mb/s.km
(Typically
Multimode Fiber)
2500 Mb/s.km
1300 nm (Lowest 1500 Mb/s.km
dispersion)
25 Gb/s.km
(InGaAsP Laser)
1550 nm (Lowest 1200 Mb/s.km
Attenuation)
Up to 500
Gb/s.km
(Best demo)
Design Considerations
• Link Power Budget
– There is enough power margin in the system to
meet the given BER
• Rise Time Budget
– Each element of the link is fast enough to meet
the given bit rate
These two budgets give necessary conditions
for satisfactory operation
Fig. 8-3: Receiver sensitivities Vs bit rate
Fig. 8-2: Optical power-loss model
PT  Ps  PR  mlc  nlsp   f L  System Margin
PT : Total loss; Ps : Source power; PR : Rx sensitivity
m connectors; n splices
Try Ex: 8.1
Fig. 8-4: Example link-loss budget
Try Ex. 8.2
Rise Time Budget
• Total rise time depends on:
–
–
–
–
Transmitter rise time (ttx)
Group Velocity Dispersion (tGVD)
Modal dispersion rise time (tmod)
Receiver rise time (trx)
t sys
1/ 2

2 
   ti 
 i 1

n
Total rise time of a digital link should not exceed
70% for a NRZ bit period, and 35% of a RZ bit period
Rise Time…
t rx  350 /Brx ns; where
Brx is receiver bandwidth in MHz
Similarly
ttx  350 / Btx ns
Assuming both transmitter and receiver as first order
low pass filters
Modal Dispersion Rise Time
Bandwidth BM(L) due to modal dispersion of a
link length L is empirically given by,
BM ( L)  Bo / Lq
B0 is the BW per km (MHz-km product) and
q ~0.5-1 is the modal equilibrium factor
t mod  0.44 / BM  440 Lq / B0 (ns)
Group Velocity Dispersion
tGVD | D | L 
Where,
D is the dispersion parameter (ns/km/nm) given by eq. (3.57)
σλ is the half power spectral width of the source (nm)
L is the distance in km
t sys
2 2q
2 2

2 2 2
440
L
 ttx  t rx  D   L 
2
B
0

Try examples 8.3 and 8.4
1/ 2
Fig. 8-6: 800 MHz-km Multimode Fiber at
800 nm, (BER=10-9)
Parameters for Fig 8-6
Power coupled
from LED : -13
dBm
Fiber loss 3.5
dB/km
System Margin 6
dB, couplers 1dB
(LED-PIN)
Dmat = 0.07
ns/(nm.km)
LED  50 nm
LASER 1 nm
Bo=800 MHz-km
q = 0.7 (modal)
Power coupled
Material
from LASER = 0 dispersion limit
dBm
with LASER is
off the graph
System Margin 8
dB (Laser-APD)
Fig. 8-7: Single Mode fiber, 1550 nm,
D = 2.5 ps/nm.km, 0.3 dB/km, two lasers
Analog Communication Links
(Amplifier
Spontaneous
Emission)
Analog (RF) links are used in
Analog TV and audio services (Legacy)
Cable modem services
Satellite base stations
Multi Channel Systems
Number of RF carriers can be summed and
directly modulate the laser
Multi Channel Systems
• These have the capability to multiplex
several RF channels
• Each RF channel is independent, it may
carry different type of data (analog video,
digital video, digital audio etc.)
• The data could be modulated onto the RF
carrier using different techniques (AM, FM,
QAM etc.)
• Nonlinearity is the major concern
Sub Carrier Multiplexing
Unmodulated (main) carrier
f2
f2
f1
f1
f0
Frequency
Sub-carriers
• Each modulating RF carrier will look like a subcarrier
• Unmodulated optical signal is the main carrier
• Frequency division multiplexed (FDM) multi channel
systems also called as SCM
Link Noise
Modal Noise: When a laser is coupled to a
multi mode fiber (MMF) modal noise
exists. To avoid this,
• Use LED with MMF
• Use a laser with large number of modes
• Use a MMF with large NA
• Use single mode fiber with laser
Modal noise at a connection of a SMF
Mode Partition Noise
• This is the dominant noise in single mode
fiber coupled with multimode laser
• Mode partition noise is associated with
intensity fluctuations in the longitudinal
modes of a laser diode
• Each longitudinal mode has different λ
• The SNR due to MPN can not be improved
by increasing the signal power
Fig. 8-12: Dynamic spectra of a laser
Laser output
spectrum vary
with time
giving
mode partition
noise
Fig. 8-16: Mode-Partition-Noise BER depends on
Receiver BER and System BER
Interferometric
Noise
due to multiple
reflections
• Increases RIN
• Laser instability
• Increases with
signal power
• Can be decreased by
having angled, low
back reflection
connectors
and isolators
Fig. 8-17: Chirping & extinction-ratio penalties